The present invention relates to a physically cross-linked copolymer and uses of the copolymer.
In today""s rapidly expanding technological world, there is an ever-growing trend for a new generation of xe2x80x9cintelligentxe2x80x9d materials. Materials can change their properties in accordance to their environment in a xe2x80x9csmartxe2x80x9d way. They have the advantage of being less limited by design constraints. One such category of these intelligent materials is xe2x80x9csmartxe2x80x9d or stimuli-responsive polymers.
Stimuli-responsive polymers have generated an explosion of interest since their relatively recent discovery due to their potential application In extraction, absorption, actuator and drug-delivery systems. These polymers are water-soluble but undergo a sharp conformational change to become water insoluble and separate out of solution, when a small change in temperature, pH or solvent composition takes place.
A thermo-sensitive smart polymer is soluble at temperatures below its lower critical solution temperature (LCST) and then undergoes a distinct phase change at the LCST, see Galaev, I. Y. et al. CHEMTECH, (1996), pages 19-25. When chemically cross-linked, such polymers give stimuli responsive polymer gels. Application of an external stimulus can cause these gels to shrink or expand to many times their volume, with an expulsion or absorption of water.
Hachisako, H. et al., Recent Res. Devel. In Pure and Applied Chem. (1998), vol. 2, pages 547-561 describes formation of a shape-remembered hydrogel upon heating, which consists of a NIPAM telomer with a stilbazole terminal group. The hydrogel redissolves very slowly when cooled to below its cloud point without stirring.
It has now surprisingly been found according to the present invention, that a novel physically cross-linked copolymer of three different monomers also has the ability of forming a shape-remembering hydrogel upon heating.
An object for the present invention is to provide a xe2x80x9csmartxe2x80x9d gel, which is not chemically cross-linked.
Another object for the present invention is to provide a xe2x80x9csmartxe2x80x9d gel, which undergoes a sharp conformational change and is soluble in water under its LCST.
Thus, in accordance with the present invention, there are provided a completely novel, non-chemically cross-linked, thermo-sensitive polymer that separates from the solution on increasing temperature to form a physically cross-linked temporary gel, to yield the shape of the holding vessel. This gel can shrink substantially but still maintain the shape of the vessel. The shrunken polymer gel can be reshaped in another form, simply by dissolution on cooling and repeating the casting process using a different shaped vessel.
Preferable the thermosensitive physically cross-linked gel is formed only at a narrowly defined combination of three monomers, 1) N-isopropyl-acrylamide NASI or 2-hydroxyethyl methacrylate HEMA, 2) styrene and 3) N-acryloyl hydroxy-succinimide.
One embodiment of the present invention is a physically cross-linked;, copolymer comprising hydrophobic monomers, hydrogen bonding monomers, and thermosensitive monomers, said thermosensitive monomers having a distinct phase change at its lower critical solution temperature (LCST) when existing as a homopolymer.
Another embodiment of the present invention is a physically cross-linked copolymer of a three component system consisting of poly (NASI,18-47-styrene10-26-NIPAAM40-63), preferable poly(NASI23-33-styrene,19-23-NIPAAM48-58).
Another embodiment of the present invention is a physically cross-linked copolymer of a three component system consisting of poly(HEMA1-8-styrene19-30-NIPAAM65-79).
Another embodiment of the present invention is use of the physically, cross-linked copolymer for casting shapes of cavities, for production of miniaturised, detailed micro-parts or micro-machine parts, or for production of thermal switches.
Yet another embodiment of the present invention is use of a physically cross-linked copolymer as drug delivery or enzyme delivery system.
Other preferred embodiments of the present invention are defined in the subclaims.
The overall structure of polymeric gels is governed by a balance, which exists between the attractive and repulsive forces of the cross-linked polymer chains. The four fundamental forces responsible for this are hydrogen bonding, Van der Waals interactions, hydrophobic interactions and ionic interactions.
Whilst these forces are balanced at lower temperatures, the polymer chains are swollen and incorporate water into the network. However, increasing the temperature disturbs this balance, increasing the attractive forces relative to the repulsive ones so that the incorporated water is forced out of the polymer network and the gel shrinks. The shrinking of the polymer gel is due to the enhanced hydrophobic intermolecular interaction of the polymer chains. The water molecules around the polymer gel have more order than those, which are more remote and hence have a lower entropy.
As the temperature is increased the gel shrinks and becomes more ordered, but at the same time the expelled water molecules become less ordered. Therefore, the resulting entropy of the overall polymer matrix is increased, forming the impetus for gel shrinkage and expulsion of water.
Chemically cross-linked, thermo-sensitive gels, are known to exist as copolymers, that possess at least one monomer component exhibiting temperature dependent aqueous solubility, Bromberg, L. E., et al. Advanced Drug Delivery Reviews, Elsevier Science B.V., (1998), 197-220.
This phenomenon was found to be true also, for a physically cross-linked polymer gel and for the inventors investigation this property was provided by N-isopropyl acrylamide (NIPAAM).
The distinctive feature for thermo-sensitive, physically cross-linked polymer gels, however, is in their ability to form reversible cross-links, and to maintain the vessel shape, on shrinkage. The two additional key monomers in the copolymer, that provide these characteristics, are a hydrophobic and a hydrophilic component, styrene and N-acryloxyl-succinimide (NASI), respectively. Styrene adds an element of hydrophobicity to the copolymer, aiding in precipitation. The hydrophilic component, N-acryloxylsuccinimide (NASI), is essential in the complementary formation of hydrogen bonds with NIPAAM, which play a key role in the cross-linking.
The composition range for the three monomers in the copolymer, where this thermo-sensitive, physically cross-linking phenomenon occurs, is quite narrow. Small deviations from this relatively narrow range disturb the necessary balance of forces, resulting in a polymer with more conventional properties.